Image forming apparatus

Abstract

A control unit changes the polarity of a potential of an intermediate transfer member in a primary transfer portion by changing the polarity of voltage to be applied to a secondary transfer member, and applies first voltage in a first step of forming an image on a recording material by transferring a toner image. A detection device detects the presence/absence of the recording material at a timing when a rear end of the toner image reaches a second position which is upstream of a secondary transfer portion on a conveyance path of the toner image. The control unit determines a timing to switch the voltage to be applied to the secondary transfer member from the first voltage to second voltage having polarity opposite to that of the first voltage, based on the detection result by the detection device.

What is claimed is: 
     
       1. An image forming apparatus, comprising:
 an image bearing member that bears a toner image; 
 an intermediate transfer member onto which the toner image is primarily transferred from the image bearing member in a primary transfer portion that contacts the image bearing member; 
 a secondary transfer member that constitutes, together with the intermediate transfer member, a secondary transfer portion, and secondarily transfers the toner image from the intermediate transfer member to a recording material in the secondary transfer portion; and 
 a control unit that controls voltage to be applied to the secondary transfer member, wherein 
 the control unit changes polarity of a potential of the intermediate transfer member in the primary transfer portion by changing polarity of the voltage to be applied to the secondary transfer member, applies a first voltage in a first step of forming an image on the recording material by transferring the toner image, and applies a second voltage having polarity opposite to that of the first voltage, in a second step, and 
 in the transition from the first step to the second step, the control unit applies the second voltage to the secondary transfer member after the recording material and the toner image pass through the secondary transfer portion in the first step. 
 
     
     
       2. An image forming apparatus, comprising:
 an image bearing member that bears a toner image; 
 an intermediate transfer member onto which the toner image is primarily transferred from the image bearing member in a primary transfer portion that contacts the image bearing member; 
 a secondary transfer member that constitutes, together with the intermediate transfer member, a secondary transfer portion, and secondarily transfers the toner image from the intermediate transfer member to a recording material in the secondary transfer portion; 
 a detection device that detects the presence/absence of a recording material at a first position which is upstream of the secondary transfer portion on a conveyance path of the recording material; and 
 a control unit, wherein 
 the control unit changes polarity of a potential of the intermediate transfer member in the primary transfer portion by changing polarity of voltage to be applied to the secondary transfer member, and applies a first voltage in a first step of forming an image on the recording material by transferring the toner image, 
 the detection device detects the presence/absence of the recording material at a timing when a rear end of the toner image reaches a second position which is upstream of the secondary transfer portion on a conveyance path of the toner image, and 
 based on the detection result by the detection device, the control unit determines a timing when the voltage to be applied to the secondary transfer member is switched from the first voltage to a second voltage having polarity opposite to that of the first voltage. 
 
     
     
       3. The image forming apparatus according to  claim 2 , wherein
 when the first position and the second position are at a same distance from the secondary transfer portion, and 
 when the detection result by the detection device indicates that the recording material is absent when the rear end of the toner image is at the second position, 
 the control unit waits until the rear end of the toner image passes through the secondary transfer portion, and then applies the second voltage to the secondary transfer member. 
 
     
     
       4. The image forming apparatus according to  claim 2 , further comprising a calculation unit that calculates time required for the rear end of the recording material to pass through the secondary transfer portion, wherein
 when the first position and the second position are at a same distance from the secondary transfer portion, and 
 when the detection result by the detection device indicates that the recording material is present when the rear end of the toner image is at the second position, 
 the control unit waits until the time calculated by the calculation unit elapses after the detection result by the detection device indicates that the recording material is absent, and then applies the second voltage to the secondary transfer member. 
 
     
     
       5. The image forming apparatus according to  claim 2 , further comprising a calculation unit that calculates time required for the rear end of the recording material to pass through the secondary transfer portion, wherein
 when the second position is located downstream of the first position with respect to the secondary transfer portion, and 
 when the detection result by the detection device indicates that the recording material is present when the rear end of the toner image is at the second position, 
 the control unit waits until the time calculated by the calculation unit elapses after the detection result by the detection device indicates that the recording material is absent, and then applies the second voltage to the secondary transfer member. 
 
     
     
       6. The image forming apparatus according to  claim 2 , further comprising a recording material length detection unit that detects a length of the recording material in a conveyance direction, wherein
 when the second position is located downstream of the first position with respect to the secondary transfer portion, and 
 when the detection result by the detection device indicates that the recording material is absent when the rear end of the toner image is at the second position, and the length of the recording material detected by the recording material length detection unit is equal to or less than a length of the toner image in the conveyance direction, 
 the control unit waits until the rear end of the toner image passes through the secondary transfer portion, and then applies the second voltage to the secondary transfer member. 
 
     
     
       7. The image forming apparatus according to  claim 2 , further comprising a recording material length detection unit that detects a length of the recording material in a conveyance direction, and a calculation unit that calculates time required for the rear end of the recording material to pass through the secondary transfer portion, wherein
 when the second position is located downstream of the first position with respect to the secondary transfer portion, and 
 when the detection result by the detection device indicates that the recording material is absent when the rear end of the toner image is at the second position, and the length of the recording material detected by the recording material length detection unit is longer than a length of the toner image in the conveyance direction, 
 the control unit waits until time corresponding to a difference between the length of the recording material and the length of the toner image and the time calculated by the calculation unit to elapse after the timing when the rear end of the toner image reaches the second position, and then applies the second voltage to the secondary transfer member. 
 
     
     
       8. The image forming apparatus according to  claim 2 , further comprising a cleaning member that contacts a surface of the image bearing member so as to clean the surface. 
     
     
       9. The image forming apparatus according to  claim 8 , further comprising a collection unit that collects toner removed from a surface of the image bearing member by a cleaning member. 
     
     
       10. The image forming apparatus according to  claim 2 , wherein
 the intermediate transfer member is an endless belt. 
 
     
     
       11. The image forming apparatus according to  claim 10 , further comprising:
 a support member that supports the belt; and 
 a voltage stabilizing element that is connected to the support member, wherein 
 a part of current supplied by the secondary transfer member to the belt flows to the ground side by the voltage stabilizing element, so that the potential generated in the primary transfer portion, which is constituted by the belt and the image bearing member, does not exceed a predetermined potential. 
 
     
     
       12. The image forming apparatus according to  claim 11 , wherein
 the voltage stabilizing element is a Zener diode. 
 
     
     
       13. The image forming apparatus according to  claim 12 , wherein
 the secondary transfer member contacts the belt at a position where the secondary transfer member faces the support member via the belt.